1. Field of the Invention
The present invention relates to a radio receiver, and particularly to a radio receiver having a test signal generating circuit used to adjust an image frequency suppression ratio (IMRR).
2. Description of the Related Art
The image frequency suppression ratio (IMRR) is an important element in a low-intermediate frequency (LOW-IF) receiver.
A television receiver, in particular, is desired to have an IMRR of −60 dB or more. In such a case, it is difficult to achieve the above IMRR unless some adjusting means is used.
Japanese Patent Laid-Open No. 2005-295470 (hereinafter referred to as Patent Document 1) and Jean Robert Tourret, et al. (NXP), “SiP Tuner with Integrated LC Tracking filter for Both Cable and Terrestrial TV Reception,” IEEE JSSC, pp. 2809-2821, Vol. 12, December 2007 (hereinafter referred to as Non-Patent Document 1) propose including a test signal generating circuit in an electronic circuit within a radio receiver, generating a test signal in an RF frequency band in the test signal generating circuit, and adjusting the IMRR using the test signal.
Incidentally, the test signal can be used also to adjust a tracking filter and an RF amplifier in an RF signal processing section of the radio receiver.
In addition, to achieve the IMRR of −60 dB or more needs sufficiently good purity (accuracy) of the test signal. This is, for example, because the phase noise of the test signal appears on an original received signal side.
A case is assumed in which at a time of adjustment of the radio receiver, the adjustment is performed by setting the test signal at an image frequency and superimposing (injecting) the test signal on the received signal of the receiver, and detecting the image-suppressed signal as an intermediate frequency (IF) signal.
In this case, supposing that a detector of the IF signal has a band of 6 MHz, the phase noise of the test signal and an image signal cannot be distinguished from each other unless the phase noise is not more than [−60−10 k×log(6M)]=−128 dBc/Hz.
It is difficult to form a voltage-controlled oscillator (VCO) of an ordinary ring oscillator type as a circuit for generating the test signal having such a phase noise. The circuit is formed by an LC resonance type VCO.